The present invention relates to an electrical discharge machining method and apparatus and, more particularly, to an electrical discharge machining method and apparatus which can realize stable servo feed machining even in a high-frequency area for finishing or the like.
In electrical discharge machining, a voltage pulse is applied between an electrode and a work (machining gap) to generate discharge therebetween, and predetermined ON time (T.sub.on) and OFF time (T.sub.off) are repeated, thereby machining the work. Generally, a time from application of the voltage pulse to the start of discharge is called a nonload voltage application time (to be simply referred to as a "nonload time" hereinafter). The ON time (T.sub.on) corresponds to a time from the start of discharge to the completion of discharge, and the OFF time (T.sub.off) corresponds to a time from the completion of discharge to the next application of a voltage pulse.
To stably maintain the discharge, the relative feeding speed between the electrode and the work need to be controlled (servo feed). As feeding speed control of this type, a method has been conventionally known in which interelectrode voltages (pulse voltages) generated in the machining gap are averaged using a filter circuit, and the feeding speed is controlled to set an average voltage to a predetermined value. Another method is also proposed in which the nonload time is directly counted by using a clock pulse or the like, and the feeding speed is controlled on the basis of the obtained nonload time (Japanese Patent Laid-Open Nos. 50-1499 and 2-109633).
In the method in which the average voltage of interelectrode voltages is detected using a filter circuit, the average voltage of the interelectrode voltages changes depending on its duty ratio (ratio of the nonload time during one period). In a machining area with a high discharge frequency (for finishing or the like), a machining method is generally used in which the nonload time is shorter than the OFF time and the like. In this case, the duty ratio becomes small, the average voltage value also largely decreases, and the resolution is degraded, resulting in difficulty in performing stable servo feed on the basis of the average voltage.
On the other hand, the method in which the nonload time is directly counted can be applied without any problem in a rough machining area with a relatively low discharge frequency. However, in a finishing area where the discharge frequency is as high as 1 MHz, the nonload time is greatly shortened. The circuit arrangement for counting the nonload time is highly complicated, and the counting precision is undesirably degraded. Additionally, in this method, the nonload time is counted and measured at a predetermined sampling period. For this reason, if the discharge frequency varies, the nonload time cannot always be accurately measured although no problem is posed at a predetermined discharge frequency.
According to the above conventional methods, stable servo feed machining cannot be performed in a high-frequency area for finishing or the like, so constant-speed feed machining is performed in many cases.